1. Field of the Invention
The present invention is in the field of inverters, and more specifically relates to an inverter particularly well adapted for converting the direct current generated by several solar arrays to a polyphase electrical current.
2. The Prior Art
In U.S. Pat. No. 3,867,643 issued Feb. 18, 1975 to Baker, et al., there is shown a programmed switching system for converting direct current into alternating current. The system employs a number of stages that are connected in series. Each stage includes a voltage source, such as a battery, and by switching, various combinations of voltages are added together. The current from each source is discontinous rather than continous as in the present invention. Further, because the voltage sources are connected in series, it is not possible to ground each of the sources, as in the present invention.
In U.S. Pat. No. 3,889,174 issued June 10, 1975 to Corry, there is shown apparatus for generating waveforms that have flat tops that extend over an interval greater than 60 electrical degrees. It can be shown that such a system requires the use of batteries as a primary source of energy. The energy is not removed from the batteries in a constant flow, and so the circuit disclosed would not be appropriate for use with solar panels.
In U.S. Pat. No. 4,052,657 issued Oct. 4, 1977 to Kleiner, et al., a circuit is shown in which power from two or more DC sources is inverted to produce two alternating current sources that are coupled to the load in a manner that causes their output voltages to be summed vectorially across the load. The resulting power source has a low impedance, which is the opposite result from that achieved in the present invention.
In U.S. Pat. No. 4,238,820 issued Dec. 9, 1980 to Naaijer, there is shown an inverter that synthesizes an alternating voltage by combining a number of direct current voltage sources in series combinations, which is contrary to the approach taken in the present invention.
In U.S. Pat. No. 4,424,557 issued Jan. 3, 1984, Steigerwald discloses the use of a phase locked loop for synchronizing the alternating current derived from a solar array through the use of an inverter with the current flowing in a utility grid. Steigerwald also shows the use of inductors and capacitors in the output of the inverter.
In U.S. Pat. No. 3,391,323 issued July 2, 1968 to Ikeda there is disclosed an inverter in which the current flow from each source is discontinuous, thereby rendering the system inappropriate for use with photovoltaic solar arrays. The voltage sources are connected in series, making it impossible to ground each source, as is done in the present invention.